A dual clutch type automatic transmission including a dual clutch having two clutch portions, two input shafts, an output shaft and a plurality of gear trains provided between the input shafts and the output shaft, generally known as one type of transmission for a vehicle, may perform a speed change operation in such a way that a torque transmission path is smoothly switched between the two input shafts by means of the clutch portions operated so as to be in an engaging/disengaging state. The dual clutch employs a friction clutch including driving side friction plates and driven side friction plates, in which the driving side friction plates frictionally engage the driven side friction plates in order to transmit the torque of the engine. Each of the gear trains includes a gear ratio for establishing one of four to seven speed stages, and the synchromesh mechanism selects an appropriate gear train so that the rotation speed is changed at the selected gear train. The operations of the dual clutch and the synchromesh mechanism are automatically controlled by means of an operation controlling portion including an electronic control device and an actuator.
A control device for the dual clutch type (e.g., twin clutch type) automatic transmission disclosed in JP2006-226380A operates so as to control the speed of the vehicle in such a way that an appropriate speed control manner is selected on the basis of a driving condition of a driving device such as an engine in order to restrain a delay of the speed change control. According to the dual clutch type automatic transmission disclosed in JP2006-226380A or the like, while the vehicle is traveling, one of the clutch portions is operated so as to be in an engaging state, and a gear train fixed to one of the input shafts is operated so as to be in an engaging state so that the torque is transmitted from the engine to the output shaft via the one of the clutch portions and the one of the input shafts. At this point, the other of the clutch portions is in a disengaging state, and the torque is not transmitted to the output shaft via the other of the clutch portions and the other of the input shafts.
The speed change operation of the dual clutch type automatic transmission generally includes a gear train changing process, a torque transmission path switchover process and a power source synchronizing process. When a speed change operation command is inputted to the control device, one of the gear trains, fixed to the other of the input shafts by which the torque is not transmitted, is operated so as to be in an engaging state in the gear train changing process. At this point, the other of the input shafts starts synchronizing with the output shaft by means of a frictionally synchronizing operation of the synchromesh mechanism.
Then, the control proceeds to the torque transmission path switch over process, and the one of the clutch portions, by which the torque is currently transmitted to the output shaft, is gradually operated so as to be in a disengaging state and at the same time, the other of the clutch portions is gradually operated so as to be in an engaging state. Accordingly, a torque transmission path through which the torque is transmitted to the output shaft is switched from the one of the input shafts to the other of the input shafts. Then the control proceeds to the power source synchronizing process. In the power source synchronizing process, the power source is synchronized with the other of the input shafts. After the power source synchronizing process, the speed change operation is terminated.
According to the known speed change operation for the dual clutch type automatic transmission, in the gear train changing process, a relatively long time is required for synchronizing the other of the input shafts, which is not transmitting the torque, with the output shaft. Accordingly, time for an entire speed change operation may be longer. For example, while the vehicle is traveling at a speed of the second shift stage established by means of the second input shaft transmitting the torque to the output shaft, in a case where an up-shifting operation is executed with up-shifting from a first shift stage established by a first shift stage gear train fixed to the first input shaft to a second shift stage established by a second shift stage year train fixed to the second input shaft, and then the shift stage is further up-shifted from the second shift stage to a third shift stage established by a third shift stage gear train fixed to the first input shaft, the first shift stage gear train fixed to the first input shaft is operated so as to be in an disengaging state, and the third shift stage gear train fixed to the first input shaft is operated so as to be in an engaging state. Specifically, in order to smoothly change the rotation speed of the output shaft, the rotation speed of the first input shaft needs to be drastically reduced from the speed corresponding to the first shift stage to the speed corresponding to the third shift stage. In the known operation, the speed of the input shaft is reduced (or increased) only by the frictionally synchronizing operation of the synchromesh mechanism, which may require more time for the synchronizing operation.
Furthermore, when the clutch portion connected to the input shaft is in a disengaging state, and the gear train fixed to the input shaft is in a disengaging state, the input shaft is idly rotating in accordance with an inertia torque, and in this condition, the rotation speed of the input shaft may be reduced due to the frictional force generated at a bearing portion of the input shaft more time is required for the synchronization between the input shaft and the output shaft. When a dry-type friction clutch not using fluid is used for the clutch portion, the clutch may not perform the synchronizing operation In the same manner as a wet-type friction clutch, which may perform the synchronizing operation effectively by virtue of the viscosity of the fluid. Accordingly, the rotation speed of the idling input shaft may be rapidly decreased using the dry-type clutch, as a result, more time is required for the synchronization between the input shaft and the output shaft.
A need thus exists for a speed control method for an automatic transmission which is not susceptible to the drawback mentioned above.